1. Field of the Invention
This invention relates to a pneumatic radial tire having an improved appearance and a low rolling resistance by using high tenacity vinylon cords as a cord for carcass ply.
2. Related Art Statement
In Japanese Patent laid open No. 60-131,302, a pneumatic radial tire is disclosed that is reinforced with a single carcass ply and having considerably improved high-speed durability, rolling resistance and cornering stability, wherein polyester cords were used as a carcass ply cord. In case of using such a polyester cord having a restricted strength, it was obliged to take a means for thickening the ply cord in order that the number of carcass plies is made one for insuring the safety of tire. Therefore, the ply cord was thickened to more than 4,500 deniers. When such a thickened ply cord is applied to the single carcass ply, the gauge of the overlap portion of the carcass ply becomes thicker in the building of the tire. This in turn unfavorably produces ruggedness in the shoulder portion of the tire to considerably damage the tire appearance when the tire is inflated under a given internal pressure.
Therefore, the greatest care must be taken in causing no overlapping of such thickened cords in the overlap portion of the rubberized carcass ply, which results in a large reduction of tire building efficiency.
Further, the use of high modulus polyester cord as a carcass ply cord has been proposed in Japanese Patent laid open No. 60-113,702. The inventors have made various reproduction experiments by varying the number of overlapped cords in the overlap portion of the carcass ply and found that the degree of ruggedness (d) in a shoulder portion 1 of a tire as shown in FIG. 1a is largely dependent upon the number of cords 2 in the overlap portion (W) of the carcass ply and the essential solution of ruggedness in the shoulder portion proposed in Japanese Patent laid open No. 60-113,702 is not attained by the use of such a high modulus polyester cord. In the reference, a relation between the number of overlapped cords in the shoulder portion and the degree of ruggedness is shown in FIG. 1b when a single carcass ply containing polyester cords of 1500 d/2 with a twisting number of 40.times.40 T/10 cm is applied to a tire of 185/70 HR 13 (internal pressure P=2.5 kg/cm.sup.2), wherein a curve 1 is the case of using usual polyester for the polyester cord and a curve 2 is the case of using high modulus polyester for the polyester cord disclosed in Japanese Patent laid open No. 60-113,702. As a result of sampling inspections, the ruggedness degree in the shoulder portion is 0.55 mm on average in case of using the usual polyester cord, while when using the high modulus polyester cord, the ruggedness degree is reduced by 0.1-0.15 mm as compared with the case of the usual polyester cord. As apparent from the above, the use of high modulus polyester cord can not basically solve the problem on the ruggedness in the shoulder portion.
Considering the fact that the degree of ruggedness is not more than 0.3 mm to solve the problem in the shoulder portion, the number of overlapped cords must be not more than 3 in case of using the high modulus polyester cord. However, the feature that such a few overlap number is obtained by the usual tire building step brings about the large reduction of the building efficiency.